The present invention relates to a dark current compensating circuit for compensating so-called "dark" current contained in photoelectric outputs of a photosensor device which are generated by scanning a plurality of photoelectric elements in which charges are accumulated.
Heretofore, automatic focus detectors for video cameras have employed a self-scanning type photoelectric element scanning circuit in which an array of photoelectric elements in which charges are accumulated is scanned in a predetermined direction and the charges of the photoelectric elements are sequentially transferred to an output. The magnitude of the photoelectric output depends on the intensity of incident light applied thereto. However, even if no light is applied to the photoelectric element, a certain amount of charge still tends to accumulate.
FIG. 1 is a graphical representation showing the photoelectric characteristic of a photoelectric element. In this graphical representation, the horizontal axis represents the intensity of light E, and the vertical axis represents the charge accumulation quantity Q. In FIG. 1, reference numerals 1, 2 and 3 designate charge accumulation lines indicating variations of the charge accumulation quantity Q. The charge accumulation times T.sub.1, T.sub.2 and T.sub.3 of the charge accumulation lines 1, 2 and 3 are reduced in the stated order as the intensity of light E increases. As the intensity of light E increases, the photoelectric element becomes saturated at the same rate. Moreover, if the charge accumulation time is long, the photoelectric element will be saturated even when the intensity of light E is small. Accordingly, even if no light is applied to the photoelectric element, charges are accumulated therein, which gives rise to a "dark" current component in the photoelectric output. The quantity of charges accumulated as the dark current component tends to increase as the intensity of light E decreases and increase with high temperature and humidity because of the physical properties of the photoelectric element. Accordingly, under conditions of high temperature and high humidity, the dark current component may be larger than the photoelectric output, thus resulting in a large error in focus detection.
In view of the foregoing, an object of the invention is to provide a photoelectric output dark current compensating circuit for use in an automatic focus detecting device which eliminates the dark current component from the photoelectric output provided by the photoelectric element.